Accept atribute on forms and links

Please consider add (optional) "accept" header to request concrete
representations:

Until now is necesary delegato on extension to download concrete
representations:

For example:

   Now: <a href="/resource.pdf"/>
   Proposed: <a href="/resource" accept="application/pdf"/>

   Now: <a href="/resource.svg"/>
   Proposed: <a href="/resource" accept="image/svg+xml"/>


The same attribute "accept" can be applied for <forms>

    <form method="get" action="/formula" accept="application/mathml+xml">
    ...

Thank you